1. Technical Field
The present invention relates to a vertical-type, integrated bipolar device and to the manufacturing process thereof.
2. Description of the Related Art
As is known, manufacture of some integrated circuits involves the use of technologies that are able to provide complementary bipolar transistors, namely NPN and PNP transistor, having very similar performances. These are readily obtainable with current technologies for the production of transistors not having a high performance as regards cut-off frequency, current-carrying capacity, and saturation voltage.
The most common and simplest way of making a PNP transistor resides in using the same layers that serve for making NPN transistors. This leads to the creation of vertical NPN transistors (as illustrated in FIG. 1) and of lateral PNP transistors (as illustrated in FIG. 2).
This solution leads, however, to NPN and PNP transistors having different characteristics.
If NPN and PNP transistors are to have comparable characteristics, it is necessary to make also the PNP transistor of a vertical type, which requires the addition of some layers (as may be seen in FIG. 3) and entails a certain complication in the design.
The manufacture of complementary bipolar transistors is also possible using SOI (Silicon-On-Insulator) substrates. As is known, SOI substrates are formed by a bottom conductive layer, in general silicon, an insulating layer, typically silicon oxide, and a top layer, of monocrystalline silicon, which houses the active structures. Examples of PNP and NPN transistors formed in a SOI substrate are illustrated in FIG. 4 and FIG. 5, respectively. As may be noted, these solutions use a trench insulation that reaches the insulating SiO2 layer. Layers dedicated for insulating the collector from the substrate and from the other components are not necessary, and the two structures are complementary.
Also these known solutions, however, are not suitable in the case where high cut-off frequencies and high operating voltages (higher than 30-40 V) are required, in so far as the transistors that can be obtained are able to withstand a BVceo voltage (collector-to-emitter breakdown voltage with open base) of just a few volts.
In general, it is difficult to obtain complementary bipolar structures having not excessive dimensions and high-frequency characteristics. In fact:                to obtain high cut-off frequencies, the emitter and the base of the transistors should be made sufficiently thin; if the base is very thin, an adequate edge structure may be needed for withstanding the voltage; a voltage higher than 40-50 V requires a BVcbo (collector-to-base breakdown voltage with open emitter) higher than 120-150 V;        an edge structure for voltages higher than 100 V (implanted ring or field plate) involves a certain bulk; and        in the complementary transistors, it may be necessary to provide well regions and buried regions for implanting dopant ions of opposite sign with respect to the original epitaxy; consequently, the collector of the transistor having the implanted regions has higher concentrations than the complementary transistor and thus different breakdown voltages.        